Fix charge balancing error and dependency housekeeping

[rkingsbury]

Summary

Fixes #141 . This was ultimately caused by a subtle problem with charge balancing when elements with multiple oxidation states (at least, as recognized by oxi_state_guesses) were present in the Solution.

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 81.37%. Comparing base (671078e) to head (70af876).
Report is 10 commits behind head on main.

❗ Current head 70af876 differs from pull request most recent head edb828c

Please upload reports for the commit edb828c to get more accurate results.

Files	Patch %	Lines
src/pyEQL/activity_correction.py	62.50%	3 Missing ⚠️
src/pyEQL/engines.py	40.00%	2 Missing and 1 partial ⚠️
src/pyEQL/equilibrium.py	33.33%	2 Missing ⚠️
src/pyEQL/solution.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
- Coverage   81.47%   81.37%   -0.11%     
==========================================
  Files           9        9              
  Lines        1425     1428       +3     
  Branches      294      295       +1     
==========================================
+ Hits         1161     1162       +1     
- Misses        224      225       +1     
- Partials       40       41       +1     

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